2.1. Introduction

Virtual 3D city and landscape models are provided for an increasing number of cities, regions, states, and even countries. They are created and maintained by public authorities like national and state mapping agencies as well as by cadastre institutions and private companies. The 3D topography of urban and rural areas is essential for both visual exploration and a range of different analyses in, for example, the urban planning, environmental, energy, transportation, and facility management sectors.

3D city models are nowadays used as an integrative information backbone representing the relevant urban entities along with their spatial, semantic, and visual properties. They are often created and maintained with full coverage of entire cities and even countries, i.e. all real world objects of a specific type like buildings, roads, trees, water bodies, and the terrain are explicitly represented. In most cases the 3D city model objects have well-defined identifiers, which are kept stable during the lifetime of the real world objects and their virtual counterparts. Such complete 3D models are a good basis to organize different types of data and sensors within Smart City projects as they build a stable platform for information linking and enrichment.

In order to establish a common understanding and interpretation of the urban objects and to achieve interoperable access and exchange of complete 3D models including the geometric, topologic, visual, and semantic data, the Open Geospatial Consortium (OGC) has issued the CityGML standard [Kolb2009]. CityGML defines a feature catalogue and data model for the most relevant 3D topographic elements like buildings, bridges, tunnels, roads, railways, vegetation, water bodies, etc. The data model is mapped to an XML-based exchange format using OGC’s Geography Markup Language (GML).

The 3D City Database (3DCityDB) is a free and open source package consisting of a database schema and a set of software tools to import, manage, analyse, visualize, and export virtual 3D city models according to the CityGML standard [YNKH2018]. The database schema results from a mapping of the object oriented data model of CityGML 2.0 to the relational structure of a spatially-enhanced relational database management system (SRDBMS). The 3DCityDB supports the open source SRDBMS PostgreSQL (with PostGIS extension) and the commercial SRDBMS Oracle (with Spatial capabilities). The 3DCityDB makes use of the specific representation and processing capabilities of the SRDBMS regarding the spatial data elements. It can handle also very large models in multiple levels of details consisting of millions of 3D objects with hundreds of millions of geometries and texture images.

The 3DCityDB is in use in real life production systems in many places around the world and is also being used in a number of research projects. For example, the cities of Berlin, Potsdam, Munich, Frankfurt, Zurich, Rotterdam, Singapore all keep and manage their virtual 3D city models within an instance of the 3DCityDB. The companies Virtual City Systems (VCS) and M.O.S.S., who are also partners in development, use the 3DCityDB at the core of their commercial products and services to create, maintain, visualize, transform, and export virtual 3D city models (see Appendix B, Appendix C, and Appendix D for examples how and where TUM, Virtual City Systems, and M.O.S.S. employ the 3DCityDB in their projects). Furthermore, the state mapping agencies of all 16 states in Germany store and manage the state-wide collected 3D building models in CityGML LOD1 and LOD2 using the 3DCityDB. In 2012 the 3DCityDB and the developer team received the Oracle Spatial Excellence Award, issued by Oracle USA.

Since the 3DCityDB is based on CityGML, interoperable data access from user applications to the database can be achieved in at least two ways:

  1. by using the included high-performance CityGML Importer/Exporter tool or the included basic Web Feature Service 2.0 in order to exchange the data in CityGML format (version 2.0 or 1.0), and
  2. by directly accessing the database tables whose relational structures are fully explained in detail within this document. It is easy to enrich a 3D city model by adding information to the database tables in some user application (using e.g. the database APIs of programming language like C++, Java, Python, or of ETL tools like the Feature Manipulation Engine from Safe Software). The enriched dataset then can be exchanged or archived by exporting the city model to CityGML without information loss. Analogously, the 3DCityDB can be used to import a CityGML dataset and then access and work with the city model by directly accessing the database tables from some application programs or ETL software.

The Importer/Exporter tool also provides functionalities for the direct export of 3D visualization models in KML, COLLADA, and glTF formats. A tiling strategy is supported which allows to visualize even very large 3D city and landscape models in geoinformation systems (GIS) or digital virtual globes like Google Earth or CesiumJS Virtual Globe.

Starting from release 3.3.0, the 3DCityDB software bundle contains the CesiumJS-based 3D viewer called “3DCityDB-Web-Map-Client” which facilitates the interactive visualization and exploration of 3D city models over the internet within web browsers on desktop and mobile computers. The most significant new functionality in release 4.0.0 is the support of CityGML Application Domain Extensions (ADEs). ADEs extend the CityGML data model by domain specific object types, attributes, and relations.

The Importer/Exporter provides a Plugin API to create further importers, exporters, and database administration tools. The software is shipped with the two optional plugins: 1) The “Spreadsheet Generator Plugin” (SPSHG) to export thematic data of 3D objects into tables in CSV and Microsoft Excel format that can be easily published as online spreadsheets (e.g., using Google Docs) and linked with the 3DCityDB-Web-Map-Client, and 2) the “ADE Manager Plugin” to dynamically extend the 3DCityDB core schema with tables and objects for storing and managing CityGML ADEs.

This documentation describes the design and the components of the 3D City Database as well as their usage for the major release 4 which has been developed and implemented by the three partners in development, namely the Chair of Geoinformatics at Technische Universität München, Virtual City Systems, and MOSS.

The development is continuing the previous work carried out at the Institute for Geodesy and Geoinformation Science of the Berlin University of Technology and the Institute for Cartography and Geoinformation of the University of Bonn.

Some figures and texts are cited from the OpenGIS City Geography Markup Language (CityGML) Encoding Standard, Version 2.0.0 [GKNH2012].